An insulated gate bipolar transistor (IGBT) comprises a four layer structure having a source region, a base or body region, a drift region and a collector region. These four regions are arranged in series and alternate in conductivity type. The source, base and drift regions structurally form an MOS field effect transistor. The collector region is provided in order to inject carriers into the drift region when the device is ON to thereby reduce the ON-resistance of the device. This device is turned ON by a gate voltage which renders a channel extending through the base region from the source region to the drift region conductive and is turned OFF by rendering that channel non-conductive.
For devices which are designed to hold off relatively large voltages such as greater than about 10-20 volts, and particularly greater than 50-100 volts, a MOSFET must have a relatively wide, lightly doped drift portion of its drain region. As a consequence, the ON-resistance of such a MOSFET device is relatively high because the device current must traverse the wide, "high" resistance drift portion of the drain region. In the IGBT the injection of carriers from the collector region into the drift region modulates (increases) the conductivity of the drift region when the device is ON by substantially increasing the mobile charge carrier density in that region. This has the effect of substantially reducing the device ON-resistance. For this reason, the IGBT is considered a preferable device to an MOSFET for high voltage devices where a low ON-resistance is desired.
Unfortunately, the advantages provided by the IGBT do not come without accompanying penalties. In particular, one problem faced with an IGBT is the fact that its four layer structure is like that of a thyristor. Consequently, if the source/base PN junction becomes forward biased, the device enters a thyristor or latched mode in which the MOS gate of the device cannot turn the device OFF. This loss of gate control results in improper circuit operation and can destroy the device. The source/base junction in an IGBT can become forward biased during normal operation if majority carrier flow in the base region (the carriers injected by the collector region) adjacent to the source region is sufficient to cause a voltage drop equal to a diode forward voltage drop across any portion of the source/base junction. Such a voltage is most likely to develop at a point remote from the base-to-source electrode contact area. The voltage drop created by majority carriers flowing in the base region results from the small but finite resistance exhibited by the base region. Therefore, a sufficient quantity of carriers and a long enough resistive path can combine to produce the necessary voltage drop (about 0.7 volts in silicon devices) to forward bias a portion of the source/base junction. The main current level at which part of the source/base junction becomes forward biased, thereby latching the device in a thyristor mode is known as the latching current of an IGBT. The latching current of an IGBT varies with the internal structure of the IGBT.
The safe operating area (SOA) of a device is a measure of its ability to turn off current in an inductive load. The boundaries of the safe operating area are limited by a combination of the voltage applied across the device and the current flowing through the device and depends on device structure and characteristics. The larger a device's SOA is, the larger the inductive current that device can turn off without damage to the device. When the device turns off while controlling an inductive load, the voltage across the device increases substantially while the inductive load prevents the current from decreasing to zero immediately. The safe operating area of the device is substantially affected by the mobile charge carrier density within the device during turn off because the presence of mobile charge carriers results in increased internal electric fields within the device structure. As a result of its bipolar nature an IGBT has a substantially greater stored charge density than a MOSFET of similar structure. Consequently, the safe operating area of an IGBT is substantially less than that of a corresponding MOSFET structure. This is a second penalty to which an IGBT is subject.
There is a need for an IGBT structure in which the ON-resistance is low, the latching current is high and the safe operating area is large.